In recent years, light-emitting organic components, in particular in the form of organic light-emitting diodes (OLEDs) have become more and more important. Besides applications in the field of displays, applications for lighting purposes also have increasingly moved to the centre of development work. The big potential of the technology in this field has been recognized generically and it is assumed that OLEDs are going to become one of the most important technologies in the field of lamps and lighting in future. Meanwhile, both the performance efficiencies and the life of components have reached a competitive level in comparison to alternative lighting technologies, such as incandescent lamps, fluorescent lamps or inorganic LEDs.
The light-emitting organic components are planar components in which a homogeneous radiation of light becomes more difficult with an increasing size of the surface area. This is in particular associated with the transparent electrode through which the radiation of light takes place and which in most cases consists of a conductive oxide, such as indium tin oxide (ITO). The surface resistivity of this electrode is typically about 20 ohm/square. At a light intensity of 1000 cd/m2 and a current efficiency of 50 cd/A, a light-emitting organic component, such as an OLED, having a square base area of 5 cm×5 cm during operation requires a current flow of about 0.05 A. If the electrodes of the component are now only contacted from one side, this results in a voltage drop of about 0.5 V. For customary light-emitting organic diodes, this voltage difference between the region of the connection to the electrode and sections of the electrode away from the connection already corresponds to a difference in light intensity which can be perceived with the naked eye.
For a light-emitting organic diode in a PIN implementation which is characterized by a particularly steep I-V curve, such a voltage difference can already account for a factor of more than 100 in the light intensity of the component. FIG. 1 shows a typical luminance-voltage curve. A light intensity of 1000 cd/m2 is achieved at 2.4 V. This shall be the mean voltage of the large-area component. This means that on one side of the lighting area, a voltage of about 2.15 V is applied whereas the voltage is 2.65 V on the other side. This corresponds to light intensities of 20 cd/m2 and 3500 cd/m2. Such differences in light intensity are not only undesired for the practical use of the light-emitting organic components but especially for lighting applications not acceptable.
To improve the lighting appearance of light-emitting organic components in the surface area, it was provided for using so-called IMI stacks (ITO-metal-ITO) instead of the material ITO in which silver is also employed besides ITO, the electrical conductivity being improved by this by a factor of about 5. However, the problem is only partially solved through this.
Alternatively, it was suggested to arrange an additional metal grid on or under the transparent electrode which reduces the surface resistivity of the transparent electrode dramatically. However, the production of such a metal grid is technically elaborate and requires more additional lithography steps within the production process. Furthermore, the metal grid reduces the active area of the light-emitting component.
Furthermore, it was suggested to form larger lighting areas by means of several small light-emitting organic components, for example OLEDs. This approach is also associated with additional costs for structuring and/or masking steps. Furthermore, spaces between the individual components remain visible and form a visual flaw.